1. Field of the Invention
The present invention relates to a semiconductor device provided with a circuit including a semiconductor element and particularly relates to a semiconductor device including a circuit configured to drive a light-emitting element.
2. Description of the Related Art
In recent years, it has been expected that self light emitting-type display devices including a light emitting element such as an electroluminescence (EL) element and/or a light emission diode (LED) element will come up as next-generation display devices. Unlike a liquid crystal display (LCD) device, the self light emitting-type display device does not include a backlight and has reduced viewing-angle dependency. Of all the self light emitting-type display devices, display devices including an organic EL element which is a current control-type light emitting element, that is, organic EL display devices have received attention. In an active matrix-type organic EL display device, a thin-film transistor (TFT) is used for a pixel circuit (hereinafter often expressed simply as a pixel) and a light emitting element that are arranged on a display area, and a circuit that is provided around the pixel circuit and that generates a signal transmitted to the pixel circuit.
For increasing the image quality of the display device, a high-precision pixel, that is, a smaller pixel has been much sought after. However, the number of TFT elements included in the pixel circuit of the organic EL display device is larger than that of TFT elements included in the pixel circuit of the LCD device in most instances. Further, since a current is passed through the organic EL element to make the organic EL element emit light, a power line should be arranged on a display area. Therefore, enough thought should be given to the way of achieving a layout effective at obtaining the high-precision pixel.
The invention of the method of connecting the source of a driving TFT configured to drive the organic EL element of each pixel circuit to a power line shared by two adjacent pixels has been disclosed in Japanese Patent No. 03922227, as an exemplary high-precision pixel layout. Since two transistors are provided through the use of a single linear semiconductor area, a high layout efficiency is obtained.
However, for reducing a pitch X1 with which pixels are arranged to respond to a further request for the high-precision pixel, a driving TFT having a large share of the pixel area should be arranged with an increased efficiency. Although a source contact pad is shared by the two pixels, as is the case with Japanese Patent No. 03922227, the length of the driving transistor provided for the two pixels does not fall within the smaller pitch of the pixel in a high-precision display device. In that case, it may become difficult to arrange the driving TFT. If the driving TFT is reduced in size, variations in the properties of elements are increased and the deterioration of the driving TFT is accelerated due to an increased current density, which decreases the quality of the display device.